The invention is directed generally to improvements in television receivers, and particularly to a system in the IF (intermediate frequency) section of a television receiver for interfacing a SWIF (Surface Wave Integratable Filter) with a tuner and IF amplifier.
A SWIF is essentially a device which converts signal energy to and from acoustic surface waves by means of transducers associated with the input and output terminals of the SWIF. The construction of the transducers themselves enables the frequency response of the SWIF to be tailored to a particular application. Hence, in television receivers, a SWIF may be employed as an IF filter. See U.S. Pat. No. 3,626,309, for example. Because a SWIF has a substantial insertion loss, the IF stage has an IF amplifier or gain block which boosts the IF signal after passage through the SWIF. Thus the SWIF is situated in the IF section between the tuner and the IF amplifier.
To interface a SWIF with a television system, attention must be paid to the so-called triple-transit effect which is common to SWIFs. This effect occurs because surface waves which are launched from the SWIF's input transducer reflect from its output transducer back toward the input transducer, which then reflect back off the input transducer and are received by the output transducer, resulting in a spurious image or "ghost" following the main image. To eliminate or tolerably reduce such reflections, the input and/or the output of the SWIF must be impedance mismatched. In this manner, the reflected waves are absorbed at the point of mismatch and the ghost effect on the reproduced television image is suppressed.
The need to suppress triple-transit reflections by impedance mismatching the input end of the SWIF with the tuner and/or the output end of the SWIF with the IF amplifier clashes with the desire to optimize power transfer into and out of the SWIF by impedance matching the SWIF with the tuner and IF amplifier. As will become evident below, this invention is directed to an improved way to comprise these directly conflicting objectives.
The prior art approaches have been to introduce an impedance mismatch at the input end of the SWIF between the SWIF and the tuner, or alternatively at the output end of the SWIF between the SWIF and the IF amplifier.
In the prior art, the most common way to achieve the impedance mismatch (and thus triple-transit reflection suppression) has been to drive the SWIF from a tuner which has a low output impedance. Because a SWIF has a high input impedance, a mismatch is created at the input to the SWIF, thereby substantially reducing the triple-transit-reflection effect. However, a significant power loss results from such a mismatch.
To avoid losing further power, the output of the SWIF has been coupled to an amplifier designed to provide a high input impedance, such as a common emitter amplifier. Thus, maximum power transfer between the SWIF and the amplifier was thought to have been obtained. It has been found, however, that the supposedly high input impedance of the amplifier is substantially reduced by the effects of stray capacitance at the interface between the amplifier and the SWIF. Consequently, the actual input impedance to the amplifier has been much lower than supposed (and desired), resulting in an additional power loss. It is known that this loss can be reduced by using a coil to tune out the stray capacitance at the SWIF-amplifier interface, but this results in a tuned-input, tuned-output amplifier configuration which is difficult to stabilize at maximum gain.
Another approach involves using a tuner with an output impedance matched to the input impedance of the SWIF (both high). The IF amplifier has a high input impedance; impedance matching network such as a pi filter is inserted between the SWIF and the amplifier. The impedance matching network has a low input impedance mismatched to the SWIF and a high output impedance matched to the input impedance of the amplifier. This approach suffers from its requirement for non-integratable external components (the inductive and capacitive components of the impedance-matching network).
For the reasons stated above, prior approaches of interfacing a SWIF with the IF section of a television receiver have been less than perfectly satisfactory, either from a cost or power-loss standpoint.